In sheet-fed printing presses and the like, for example, sheets are fed one by one from a sheet feed apparatus to a printing unit, subjected to printing, and then discharged to a sheet discharge apparatus. Here, if a plurality of sheets are overlapped and fed from the sheet feed apparatus to the printing unit, printing defects occurs, and wasted sheets are produced accordingly.
In this respect, the feed of sheets is made temporarily stoppable by employing, for example, a sheet feed safety device (see Patent Literature 1 listed below or the like, for example), a detection device (see Patent Literature 2 listed below or the like, for example), or the like. In the sheet feed safety device, a pair of wheels are arranged to sandwich the path line of a traveling sheet, and one of the wheels is supported vertically movably. When a plurality of sheets accidentally overlapped pass between the wheels, the resultant vertical movement of the one wheel swings an arm coupled to the shaft of the wheel. The swing of the arm moves the working end of a potentiometer, thereby detecting that the plurality of sheets are accidentally overlapped and fed. In the detection device, an operating pin and a contact lever are swung to make electric contacts touch each other, so as to detect whether a plurality of sheets are accidentally overlapped and fed.